1. Field of the Invention
This invention relates generally to a method of determining a response characteristic of a microwave device, and to analogous apparatus. More particularly, the invention relates to apparatus comprising microwave resonators together with precision timing equipment and methods of using these for calibrating the amplitude response in microwave equipment and components.
2. Related Art
The requirement exists for calibrating the amplitude response of measurement equipment and components for use in the microwave region of the electromagnetic spectrum (i.e. preferably in the frequency range 1 to 300 or even 1000 GHz). In such equipment and components the output signal may be attenuated or amplified from an input signal and the ratio of output to input (the gain or attenuation) needs to be accurately known.
General methods of measuring attenuation are described by F. L. Warner in "Microwave Attenuation Measurement" (Published by Peter Peregrinus, London 1977).
Precision microwave attenuation standards are traditionally made from resistance ratios which are carefully constructed to be frequency independent. At the highest level of precision such ratios may be compared with the National Standard. A variety of methods are used for National Standards for microwave attenuation and these are reviewed by H. Bayer et al. "Attenuation and Ratio--National Standards", Proc. IEEE 42, page 46, 1986. The UK primary standard is currently based on a waveguide beyond cut-off calibrator (See: R. W. Yell, "Development of a High Precision Waveguide Beyond Cut-off Attenuator", CPEM Digest 1972 pp 108-110). In such standards, the microwave frequency is often down-converted in a linear mixer to a lower intermediate frequency (IF) and measurements made at the IF with high precision mechanical apparatus. Such apparatus is capable of calibration to high resolution, for example to 0.0002 dB in 100 dB, with an accuracy of 0.01 dB in 10 dB, but is complex and costly to construct. A common feature of known calibration techniques is that they calibrate the steady-state response of the device under test.
The invention described herein preferably uses high Q microwave resonators, where Q is defined by: EQU Q.sub.1 =f.sub.O /.DELTA.f
Where Q.sub.1 is the loaded Q of the resonator, f.sub.O is the resonant frequency of the resonator and .DELTA.f is the bandwidth of the resonance at the half power (3dB) points. `High` Q in the context of this invention is Q&gt;10.sup.4, 10.sup.5 or 10.sub.6. As of 1995, Q's as high as 10.sub.11 are achievable practically. Typical resonators known in the art include dielectric resonators (See: D. G. Blair et al. "High Q Microwave Properties of a Sapphire Ring Resonator", J. Phys. D: Applied Physics, Volume 15, Page 1651, 1982) and superconducting resonators (See: V. B Braginskii et al. "The Properties of Superconducting Resonators on Sapphire", IEEE Trans. on Magn. Volume 17, Page 955, 1981, and C. D. langham and J. C. Gallop, "High Stability Cryogenic Sapphire Dielectric Resonator", IEEE Trans. Instrum. and Meas., 42, Page 96, 1993).
Superconducting resonators fabricated from low temperature superconducting (LTSC) materials can demonstrate Q in the range 10.sub.6 to 10.sub.11 but such devices must operate at a low temperature, typically less than 4.2K (-269.degree. C.), using liquid Helium as coolant.
Development of high temperature superconducting (HTSC) oxide materials has allowed fabrication of high Q resonators (Q&gt;10.sup.6) from these materials which can operate at 77K (-196.degree. C.) using liquid Nitrogen coolant. Different designs of resonator are known (See: S. J Fiedziusko et al. European Patent Application (EPA) 0496512A1, K. Higaki et al EPA 0522515A1 and EPA 0516145, and Z-Y Shen, PCT Application PCT/US92/09635) using different arrangements to make best use of HTSC materials properties.